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In an interview of Patricia Guerri, teacher of Saito Sensei's lineage in Paris, published in the french Aikidojournal, I read a paragraph telling a story I had not heard about before :

(my translation, original reference : http://www.aikidojournal.eu/docs/50/54_946_fr.pdf , page 21)
But what happened when he (Morihei Ueshiba) settled in this remote corner of Japan (Iwama), which is, it must be said, infested with yakuza, the Japanese mafia ?
It was their lair and they saw a man arrive and wanting to create a dojo. They came to see the master to extort him money : "This will cost you so and so, or else...". In front of this persistent threat, do you think he filed a complaint at the police station ? So with Saito Sensei they "cleaned up the place". Fights took place with knives and broken bottles. They had to impose themselves otherwise it was over for aikido in Iwama. The yakuza are not easy people. O Sensei and Saito Sensei stood their ground. They did not avoid the fight. And little by little the yakuza went away. Their core group, those who stayed, learned to respect the master and became his friends : so was born the harmony between the yakuza and O Sensei !
(end of translation)

Is there any other information confirming or giving more details about this episode ?

When I first came to Japan, I knew no Japanese and so, on my first visit to the Aikikai Hombu, I was accompanied by an English-speaking colleague from Hiroshima University. My colleague was an expert on Japanese social studies and religion, but had no experience of aikido. We entered the Hombu, were welcomed by Masatake Fujita, had a private meeting with Kisshomaru Doshu in his house next door, and were given a tour of the building. After leaving the Hombu I asked my colleague for his reactions, and was astonished by his very sharp response: "They are a bunch of gangsters!"

Later, after I got to know him better, my colleague expanded a little. After the war, the Japanese martial arts were regarded as heavily right wing (therefore, bad) and no upstanding Japanese would consider the prospect of marrying his daughter to a martial arts expert. Right-wing groups and yakuza (暴力団 : 'violent groups', in Japanese) were regarded as much of a muchness and all the Japanese I have met and got to know have developed an efficient antenna for sensing yakuza a mile away. They were people no self-respecting Japanese would associate with. The martial arts were tarred with the same brush, but to a lesser extent.

Even now, the Aikikai Hombu is associated with the Japanese political 'establishment', which is well to the right of center.

So the story does not surprise me, but I wonder what the yakuza groups in Iwama were doing in 1942, when Morihei Ueshiba first moved there.